双流环控制的单相逆变器并网 外文文献翻译 毕业设计（论文）

1、附录 英文翻译1. 英文文献原文Control of inverter-based micro-grids T.C. Green, M. Prodanovic Imperial College London, Department of Electrical and Electronic Engineering,Control and Power Group, London SW7 2AZ, United KingdomAvailable online 28 September 2006AbstractThe predicted growth of small-scale non-50/60

2、Hz power sources and the desire to be able to support loads independently of the public electricity grid requires the development inverter-based micro-grids. Power electronic interfaces have very different characteristics to conventional electrical machines and, therefore, different operation, contr

3、ol and protection schemes are required. Attention also needs to be given to the dominance of single-phase harmonically distorting loads in some networks and control schemes put in place that maintain voltage quality. A control scheme that exploits the controllability of inverters to operate a micro-

4、grid and provide good power quality is examined and compared with both traditional power systems and with control of dc/dc power converters. The limitations of communication and control bandwidth are discussed. Experimental results are used to illustrate the performance that can be achieved with var

5、ious combinations of linear and non-linear, three-phase and single-phase loads. 2006 Elsevier B.V. All rights reserved.Keywords: Distributed generation; Micro grids; Inverters; Power quality1. Introduction Micro-grids are thought to be a likely direction for evolution of power systems that incorpora

6、te distributed generation. The term micro-grid is not strictly defined and covers a wide range of possible systems 13. Here, the term is taken to mean a sub-set of a power distribution system that contains sufficient energy sources to supply most or all of the local load. The micro-grid is thus able

7、 to operate with only a small power exchange with the rest of the system and at times can separate from the public system and run as an independent island. A micro-grid differs from existing island power systems (both physical islands and electrical islands, such as offshore oil/gas platforms, ships

8、 and aircraft) in that connection to and disconnection from a public grid is a regular event. It is anticipated that the loads will be nor-mal domestic and commercial consumer loads. As such, large numbers of single-phase diode-rectifier loads are anticipated and the supply current is expected to be

9、 unbalanced and distorted. In some island systems, for instance an oil platform, the initiation of major loads (hoists, pumps, etc .) can be anticipated and the generation resources can be put in readiness for the transient. This may be possible in some sorts of micro-grid but, if not, load forecast

10、ing is likely to be important and demand-side participation in control may be needed. The generating technology found in a micro-grid may take many forms and an individual micro-grid is likely to include a mixture of these. Sources may be small, such as domestic CHP systems or roof-top photovoltaic

11、panels or relatively large, such as office-block-scale CHP using gas-turbines or reciprocating engines. Sources may be intermittent (photovoltaic and heat-led CHP) or fully controllable (electric-led CHP or simple diesel/gas fuelled generators). To make best use of intermittent sources and/or to pro

12、vide security, a micro-grid may include a storage element, such as a flywheel, battery or hydrogen electrolysis/fuel-cell system. Relatively large generators, especially existing peak-lopping diesel generators that are turned to micro-grid duty, are likely to be conventional synchronous machines gen

13、erating at 50/60 Hz three-phase. Newer technology will be different. Some sources are natural dc sources, such as photovoltaics, fuel-cells and batteries. Others are variablefrequency or high frequency ac, such as micro-turbines and fly-wheels. The non-50/60 Hz sources will need to be interfaced to

14、the grid via inverters. The form the micro-grid takes and the type of loads sup-plied will have a large impact on the operating and control regime of the system. As yet, there are no established standard operating schemes and it may never be possible or desirable to have a universally applied method

15、. This paper will describe the operation and control of a micro-grid for a particular situation, namely, a micro-grid that is formed of a relatively small number of generation sources that are interfaced to the grid through inverters and expected to supply unbalanced and distorting load current.2. T

16、he power parkThe example micro-grid system will be a power park. A power park consists of several power sources in relatively close proximity used to improve the electricity service to a group of customers. The customers may be a retail park, shopping mall or group of office buildings. If these are

17、new consumers the power park may be motivated by network congestion or powershortages. Other motivations might be the wish to install cogeneration or to provide premium power. The premium aspect would be the ability to continue supplying high value customers in the event of outage of the public grid

18、 or voltage sag. It might also mean avoidance of other power quality problems, such as waveform distortion, flicker or swell. The improved power quality might come about from a lowered supply impedance, active management (such as active filtering) and/or disconnection from the public grid when the g

19、rid is deemed to be the source of the problem. Because some system operators presently do not allow distributed generators to control the voltage at the point of common connection, some of the power quality benefits may only be available when the micro-grid is running as an island. For increased rel

20、iability, flexibility and efficiency (when running part-load) several generators are required.The power park might be owned by a landlord, a building ser-vices company, the consumers themselves or the existing utility company. Power export to the public grid might or might not be used whatever the o

21、wnership of the generation. Micro-grids in general may have a variety of owners of generation but the power park variant is likely to see a single owner or a single operator. Further, the generators will be in groups that are physically and electrically close together. The generation technology may

22、be a mix of traditional electrical machines and inverters. There are several possible operating modes of the power park:(1) Grid-connected operation with fixed local generation: The grid connection will see a fluctuating power flow as loads change. The power flow could be in either direction.(2) Gri

23、d-connected operation with load following local generation: The grid will see a fixed power flow which might be import (to cover short fall of local generation), export (to sell excess capacity) or zero (floating of the micro-grid but with a connection maintained to cover contingencies).(3) Island o

24、peration : The local generation will load follow, possibly using storage or demand-side participation to increase security. The operation and control issues to be faced are:(1) How to share steady-state load between generating sets?(2) How to share transient load between sets?(3) How to switch seaml

25、essly from grid-connected to island operation?(4) How to incorporate additional features, such as active management of waveform quality?3. Characteristics of electrical machines and inverters The history of power system generation is the history of the synchronous machine and the characteristics of

26、this machine have influenced the development of other aspects of the system. For instance, protection schemes rely on the short-term over current rating of an electrical machine (resulting from its large thermal mass) to provide the fault current necessary to discriminate faults. In turn, the requir

27、ed clearance times for faults are dictated by the over-swing and loss of synchronism characteristics of machines. On the other hand, electronic power converters are well established in the fields of OEM power supplies and industrial drives but have not influenced the development of traditional power

28、 systems. Electronic power converters have very different characteristics to electrical machines and would have directed engineers toward a very different transmission and distribution system.The relevant characteristics of electrical machines are:(1) Operate as voltage sources whose amplitude can b

29、e adjusted. The adjustment is normally part of a closed-loop excitation control scheme with a relatively low bandwidth.(2) Sine-wave voltage is a feature designed into the construction of the machine. The total harmonic distortion (THD) of the voltage is low. Distortion in the load current will incr

30、ease the distortion in the voltage to an extent dependent on the source impedance.(3) The short-circuit current is high because the source impedance is low and no current limiting is employed.(4) The current rating is set by temperature rise of the winding insulation. The thermal time-constant of th

31、e winding and surrounding steel is relatively large and a useful short-term rating is available. Fault currents of as much as 10 times the steady-state maximum can be sustained for several mains cycles.(5) Real power exchange is dictated by the torque applied to shaft. Steady-state load sharing can

32、be applied by a closed-loop governor setting that makes power output a function of the (common) system frequency. This is a designed in feature of the governor system but it is similar to the natural tendency for the speed of the prime mover to droop when electrical load is drawn.The corresponding c

33、haracteristics of inverters are:(1) Operate as voltage sources (although current source versions are known) with near instantaneous and independent control of the magnitude of each phase.(2) Sine-wave voltage can be achieved through use of a suitable reference waveform and modulator but any shape ca

34、n be used at will. Alternatively, closed-loop current regulation can be applied to achieve various current waveforms. The low-frequency spectrum of an inverter is well controlled but the switching action of the inverter produces high frequency distortion that can only be addressed through filtering.

35、(3) Potential short-circuit current is high but protection against it must be provided in the form of current limiting action.(4) The current rating is set by the temperature rise of the semiconductors. The thermal time-constant of the semiconductor is very short and large over currents cause device

36、 failure in considerably less than 1 ms. The cooling system also has a relatively short thermal time-constant and so even moderate-over currents can only be tolerated for short periods unless the inverter has been over-rated to accommodate them.(2) Real power exchange is dictated by the references a

37、pplied to the control system (subject to the dc-link being able to source or sink this power). There are several low-power application areas where electronic power converters are used for power distribution, such as in telecommunication systems and large computers 4. These power systems often consis

38、t of several dc power supplies running in parallel. The systems that have developed here are quite different from public ac power grids. The operating schemes are hierarchical control-loops that actively share power between modules through distribution of current demand signals. The current demand s

39、ignals are arranged to ensure that modules are not subjected to fault currents beyond their ratings and the over-all system has a limited fault current that causes collapse of the system voltage. There are several low-power application areas where electronic power converters are used for power distr

40、ibution, such as in telecommunication systems and large computers 4. These power systems often consist of several dc power supplies running in parallel. The systems that have developed here are quite different from public ac power grids. The operating schemes are hierarchical control-loops that acti

41、vely share power between modules through distribution of current demand signals. The current demand signals are arranged to ensure that modules are not subjected to fault currents beyond their ratings and the over-all system has a limited fault current that causes collapse of the system voltage. of

42、the high order filters, is likely to lie near the upper limit of the controller bandwidth. This is problematic. There is barely enough control bandwidth to suppress the filter resonance. Placing the resonance safely above the controller frequency limit is not feasible because insufficient filter att

43、enuation would be achieved. Reducing the resonant frequency to well within the control bandwidth will require physically large components and will make the system response sluggish above that frequency.4. Control of an inverter for power export4.1. Grid-connected modeIn grid-connected mode, the obje

44、ctive is to export a controlled amount of power into the established voltage. This mode of operation has been extensively examined for power flow predominantly in the opposite direction under the guise of active rectifiers 5. Most active rectifiers employ simple first-order filters (because power qu

45、ality in the medium frequency range may not be particularly important) but second-order filters are also reported. Control of the exported power is through control of the in-phase component of current. Phase-locked loop (PLL) techniques are used to ensure synchronism 6. Control in dq-axis (synchrono

46、us reference frame) form is usually preferred to magnitude-angle form. The current demands are generated from the power demands using the local voltage magnitude (available from the PLL). The power demands themselves come from the micro-grid supervisor (MGS) with the exception of photovoltaic and he

47、at-led CHP sources which generate autonomously. The MGS will operate either to run the generators at fixed output (presumably optimum in some sense) or load following so as to regulate export from or import to the micro-grid. The overall control structure will be similar to Fig. 3. The current is co

48、ntrolled in closed-loop form. The current references (d- and-q axes) are set on the basis of the required real and reactive powers. Strictly speaking, the power is set open-loop but the closed-loop current control ensures proper output power levels and power sharing between parallel units. Fig. 3 sh

49、ows some standard inverter control techniques in addition to the simple current-control-loop. These are the addition of decoupling terms to compensate the natural coupling between d- and q-axes (introduced by the filter reactance) and the feed-forward of the capacitor voltage into the current contro

50、l-loop. For clarity, Fig. 3omits terms that might be added to compensate variation of the dc-link voltage of the inverter. The grid voltage may not be available for monitoring so alternative connections for the PLL are shown. Because each inverter controls its output current, current sharing and pow

51、er sharing are assured according to the demands established by the MGS. The design of the controller and the design of the filter need to be considered together in order to establish good attenuation of both high and low-frequency interference 7. Controllers implemented in discrete time will also re

52、quire additional decoupling terms 8. Public electricity grids (at distribution voltage levels) often have significant harmonic distortion of the voltage. Choosing to control the output current of the inverter in an explicit current control-loop has the advantage of making the inverter a high impedan

53、ce path that prevents flow of harmonic currents due to grid voltage distortion. (The filter capacitor does provide a path for harmonic currents and this must be account for in selecting its value.) In contrast, a voltage source synchronous machine will present a low-impedance path to non-triplen har

54、monics. optional connections). Thought must be given how the voltage magnitude signal is used in the power calculation. The current calculated as necessary to export constant power will be harmonically distorted if the voltage is harmonically distorted. It may be preferred low-pass filter the calcul

55、ated current reference to remove harmonic terms. The consequence of this is that the power export will contain ripple at frequencies related to the voltage harmonics. In other words, if the voltage is distorted there are three choices: (i) export undistorted current but distorted power, (ii) export

56、undistorted power but distorted current or (iii) suffer some degree of distortion to both current and power. Fig. 3 shows the current demand fed through a low-pass filter to ensure undistorted current references (option (i).It would be possible to set all inverters to export balanced fundamental-fre

57、quency-only current. However, the single-phase diode-rectifier loads will still required unbalanced and harmonic current components and these will have to be provided by any machines on the micro-grid or by the public grid. Drawing these components through significant line impedances will worsen the

58、 voltage distortion. Drawing a high proportion of distortion current from the public grid may be disallowed by the utility operator or by regulations. To achieve good voltage quality for local consumers extra control functions can be added. The voltage at the filter capacitor can be sensed and filte

59、red to identify the harmonic distortion. This feedback is used to generate a current reference to add to the active and reactive current references in order to correct the distortion. (The terms reactive current and reactive power are here used to refer to the quadrature component of fundamental frequency current and its associated reactive power.) Fig. 4 shows a suitable control structure. It re